This invention relates generally to television signal processing circuitry, and more particularly the invention relates to circuitry for ghost signal cancellation in television signals in all broadcasting standards and in HDTV.
Echo signals or ghost signals have been one of the major problems in modern television transmission. When a transmitted signal is received from the air, ghosts can be caused by reflections from mountains, buildings, and the like. In a television signal received through cable, the ghosts can be due to discontinuities of the connectors. Although the ghosts are generated in a radio frequency spectrum, the process in which the ghost is created can be accurately modelled as a linear distortion in a baseband signal. Therefore, cancelling the ghosts can be accomplished by passing the baseband signal through a linear filter which is an inverse of the linear model of the ghosting process.
Prior efforts in eliminating ghosts used analog technology such as a charge coupled device (CCD) transversal filter which did not produce significant improvement due to inaccuracy of the filter. More recently, a ghost canceller reference (GCR) signal has been proposed for use in training of filters. Digital filters have become widely accepted as alternative methods to implement the ghost cancellers.
Ghost signals can arrive at a receiver before and after the main signal, resulting in a precursor ghost and post-cursor ghost, respectively. A filter to cancel the precursor ghost is a physically non-realizable infinite impulse response (IIR) filter, which can be approximated by a longer finite impulse response(FIR) filter. The post-cursor ghost can be cancelled by an IIR filter. Therefore, the typical digital filter to cancel the ghost signals is an FIR feedforward filter followed by an IIR feedback filter.
In order to calculate the filter coefficients, a ghost canceller reference (GCR) signal is transmitted from the broadcasting station, typically in one line of the vertical blanking interval (VBI). The coefficient of the filters can be estimated by comparing the received GCR signal with a stored standard GCR signal. In order to cancel the unknown ghosts, the filters have to be able to adapt to different situations. Adapting the feed-forward section is straightforward since well-known algorithms such as the mean squared error (MSE) algorithm or zero forcing (ZF) algorithms can be used to estimate the FIR filter coefficients. However, adapting the feedback filter coefficients directly using the same algorithms can result in an unstable filter. Although a stability check to abandon an unstable filter while designing another filter, this involves a significant amount of extra computations and may end up to a suboptimal solution.
Accordingly, a primary object of the present invention is a stable television ghost cancellation system including a feed-forward filter and a feedback filter with a stable and optimal adaptation of the feedback section of the ghost canceller.